The present invention relates to a blue laser diode and, more particularly, to a two section blue laser diode with an amplifier region and a modulator region to reduce power output variations.
Solid state lasers, also referred to as semiconductor lasers or laser diodes, are well known in the art. These devices generally consist of a planar multi-layered semiconductor structure having one or more active semiconductor layers bounded at their ends by cleaved surfaces that act as mirrors. The semiconductor layers on one side of the active layer in the structure are doped with impurities so as to have an excess of mobile electrons. The semiconductor layers on the other side of the active layer in the structure are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. Layers with excess electrons are said to be n-type, i.e. negative, while layers with excess holes are said to be p-type, i.e. positive.
An electrical potential is applied through electrodes between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers across the p-n junction so as to “inject” them into the active layers, where electrons recombine with holes to produce light. Optical feedback provided by the cleaved mirrors allows resonance of some of the emitted light to produce coherent “lasing” through the one mirrored edge of the semiconductor laser structure.
Semiconductor laser structures comprising group III-V nitride semiconductor layers grown on a sapphire substrate will emit light in the near ultra-violet to visible spectrum within a range including 360 nm to 650 nm.
The shorter wavelength of blue/violet laser diodes provides a smaller spot size and a better depth of focus than the longer wavelength of red and infrared (IR) laser diodes for laser printing operations and high density-optical storage. In addition, blue lasers can potentially be combined with existing red and green lasers to create projection displays and color film printers.
The III-V nitrides make possible diode lasers that operate at room temperature and emit shorter-wavelength visible light in the blue-violet range under continuous operation. The III-V nitrides comprise compounds formed from group III and V elements of the periodic table. The III-V nitrides can be binary compounds such as gallium nitride (GaN), aluminum nitride (AlN), or indium nitride (lnN), as well as ternary alloys of aluminum gallium nitride (AlGaN) or indium gallium nitride (InGaN), and quartemary alloys such as aluminum gallium indium nitride (AlGaInN).
These materials are highly promising for use in short-wavelength light emitting devices for several important reasons. Specifically, the AlGaInN system has a large bandgap covering the entire visible spectrum. III-V nitrides also provide the important advantage of having a strong chemical bond which makes these materials highly stable and resistant to degradation under high electric current and intense light illumination conditions that are present at active regions of the devices. These materials are also resistant to dislocation formation once grown.
High speed and high resolution printing requires laser devices with little or no fluctuations of the output power. For example, the variation in the laser light output required for red and IR laser diodes for printing applications is smaller than 4% and those requirements would be similar for AlGaInN laser diodes.
Heat is generated through voltages drops across the metal electrode/semiconductor interfaces, which have a finite resistance, and through voltage drops across the resistive semiconductor layers. Energy is also introduced into the active region of the laser by injecting electrons into the conduction band and/or holes into the valence band. Electrons relax into the lowest energy state of the conduction band and holes relax into the lowest energy state of the valence band through non-light emitting processes and release their energy in the form of heat.
When a laser device is switched from the OFF to the ON state, transient heating, or heating that changes over time, can cause the light output of AlGaInN laser diodes to drop significantly.
As an illustrative example, an AlGaInN blue laser diode is forward biased with a constant current above the lasing threshold current. At the initial time t=0, with a constant current of 65 mA, the blue laser diode will have a first output power PI of 9.5 mW with a laser structure temperature of 20 degrees C., as shown in FIG. 1.
However, as time increases with the blue laser diode above lasing threshold with the constant current, the temperature of the laser structure increases. This increased temperature results in a decreased output power for the AlGaInN laser diode.
At a subsequent time t=∞, still with a constant current of 65 mA, the blue laser will have a second output power P2 of 6.2 mW with a laser structure temperature of 30 degrees C., as shown in FIG. 1. The second output power P2 is lower than the initial output power P1. Thus, the plot of output power versus time of FIG. 2 shows an initial output power of P1 at turn-on, “drooping” to the second lower output power P2 as the blue laser diode is operated.
Thermal fluctuations are especially deleterious to maintaining constant optical power output, especially during pulsed modulation. In virtually all of the applications of these lasers, it is necessary to modulate the output of the laser into a series of pulses.
Transient heating during a sequence of pulses can have a cumulative effect on the temperature depending on the number and frequency of the pulses. For example, if the time between successive pulses is large, the laser diode will be given sufficient time to cool, so that the application of the driving current has a large temperature effect (i.e., a large droop in output power will occur at turn-on of the next pulse). The shorter the time between pulses, the less time the laser diode has to cool between one pulse and the next, leading to a sustained increase in the temperature of the laser. This sustained temperature increase results in a further decrease in the output pulse obtained with a constant level of input current.
Another related consequence of transient heating of a laser is wavelength variation during a pulse and over long streams of pulses. Essentially, the operating wavelength of a laser diode is dependent on the temperature of the laser diode. If the temperature varies, the wavelength of operation will vary. The effect of this variation of wavelength, for example in the laser xerography application, is to vary the energy that can be written onto the photoreceptor. This can also translate directly into variations in the spot size and pattern on the photoreceptor.
Digital printing requires accurate control of the optical energy delivered in each pulse. In systems currently known to those skilled in the art, a predetermined amount of energy is delivered in each pulse by turning on the optical beam to a desired power level for a fixed time interval. This approach requires that the laser output power be reproducible from pulse to pulse and constant during a pulse, in order that the optical energy delivered in each pulse be accurately controlled. Accurate control is especially important in printing with different grey levels formed by varying the number of exposed spots or when exposing very closely spaced spots in order to control the formation of an edge.
Due to the poor thermal conductivity of the sapphire substrate and the relatively high electric power consumption of III-nitride baser laser devices, transient heating is an issue to AlGaInN devices. For example, AlGaInN laser devices have threshold currents in the order of 50 mA and operating voltages of 5 V (compared to about 15 mA and 2.5 V for red lasers).
It is an object of the present invention to provide a blue laser with reduced power output variations due to transient heating.